Accurate control of mass flow rates is a requirement in many industries. For example, in the process industry, gas flow is controlled. In the gas turbine industry, mass flow rate is used to meter fuel into gas turbines. Mass flow is determined from the equation {dot over (m)}=ρVA where {dot over (m)} is the mass flow rate, ρ is the gas density, V is the velocity of the gas, and A is the cross-sectional area where the gas is flowing.
Upstream pressure and temperature measurements are used to derive the gas density. To measure the velocity in subsonic valves, downstream pressure is also measured and velocity is derived based on the pressure differential of the upstream and downstream pressures. However, the downstream pressure measurement reduces the accuracy and reliability of the flow control due to the use of both upstream and downstream sensors.
As a result of the reduced accuracy and reliability, the industry developed sonic gas valves where the velocity in the throat (narrowest section) of the nozzle of the valve is Mach 1.0. When the gas velocity is Mach 1.0 in the throat, downstream pressure signals cannot propagate upstream through the nozzle throat because pressure signals cannot travel faster than the speed of sound. One result of this fact is that the upstream flow into the nozzle is not affected by downstream pressure when the velocity in the nozzle throat is Mach 1.0. Hence, even when the downstream pressure is lowered, the velocity in the nozzle throat is not affected. As a result, downstream pressure measurements no longer are required to determine velocity.
Achieving sonic flow (i.e., gas velocity is Mach 1.0) is more easily achievable when the valve inlet pipe is in-line with the outlet pipe (e.g., the center-lines are co-linear). However, the valve inlet pipe and outlet pipe are not in-line in many installations. In valves where the inlet pipe is orthogonal to the outlet pipe, the gas flow pattern essentially turns ninety degrees from inlet to outlet. The flow coming in from a side rather than the centerline of the discharge pipe causes the flow in the valve to be non-uniform around the valve nozzle throat. As a result, sonic flow is more difficult to achieve and higher pressure drops are required to achieve sonic flow. The higher pressure drop may account for significant energy loss and adversely affect the efficiency of the system.